Use of fat mass and fat free mass standard deviation scores obtained using simple measurement methods in healthy children and patients: comparison with the reference 4-component model

Abstract

Background: Clinical application of body composition (BC) measurements for individual children has been limited by lack of appropriate reference data.

Objectives: (1) To compare fat mass (FM) and fat free mass (FFM) standard deviation scores (SDS) generated using new body composition reference data and obtained using simple measurement methods in healthy children and patients with those obtained using the reference 4-component (4-C) model; (2) To determine the extent to which scores from simple methods agree with those from the 4-C model in identification of abnormal body composition.

Design: FM SDS were calculated for 4-C model, dual-energy X-ray absorptiometry (DXA; GE Lunar Prodigy), BMI and skinfold thicknesses (SFT); and FFM SDS for 4CM, DXA and bioelectrical impedance analysis (BIA; height(2)/Z)) in 927 subjects aged 3.8-22.0 y (211 healthy, 716 patients).

Results: DXA was the most accurate method for both FM and FFM SDS in healthy subjects and patients (mean bias (limits of agreement) FM SDS 0.03 (± 0.62); FFM SDS -0.04 (± 0.72)), and provided best agreement with the 4-C model in identifying abnormal BC (SDS ≤-2 or ≥ 2). BMI and SFTs were reasonable predictors of abnormal FM SDS, but poor in providing an absolute value. BIA was comparable to DXA for FFM SDS and in identifying abnormal subjects.

Conclusions: DXA may be used both for research and clinically to determine FM and FFM SDS. BIA may be used to assess FFM SDS in place of DXA. BMI and SFTs can be used to measure adiposity for groups but not individuals. The performance of simpler techniques in monitoring longitudinal BC changes requires investigation. Ultimately, the most appropriate method should be determined by its predictive value for clinical outcome.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Bland-Altman analyses for the agreement between the 4C model and (A) DXA, (B) BMI, (C) bicep SFT, (D) tricep SFT, (E) subscapular SFT and (F) suprailiac SFT in the measurement of FM.

SD scores (SDS) were calculated using ICH reference data . Difference in SDS between techniques was calculated by subtracting 4C FM SDS from the FM SDS given by the ‘simpler’ method. Horizontal lines on the graph indicate the mean difference between techniques, and two standard deviations above and below this value. Different symbols indicate which study group each subject was from, as shown in the key. 4C model; four-compartment model, DXA; dual-energy X-ray absorptiometry, BMI; body mass index, SFT; skinfold thickness. DX; healthy children, PT; young adults born preterm, OB; obese children, GSD; glycogen storage disease, ALL; acute lymphoblastic leukaemia, CF; cystic fibrosis, ED; eating disorders. Evaluation of SFT as a measurement of FM was carried out using a limited population consisting of study groups DX, PBC and CF. The OB, GSD, ALL and ED studies did not include SFT measurement as part of their protocols. Since no subjects were available for the overweight disease group, normal subjects were compared to underweight subjects only.

Figure 2. Bland-Altman analyses for the agreement between the 4C model and (A) DXA and (B) BIA in the measurement of FFM.

As for Figure 1, SD scores (SDS) were calculated using ICH reference data . Difference in SDS between techniques was calculated by subtracting 4C FFM SDS from the FFM SDS given by the ‘simpler’ method. Horizontal lines on the graph indicate the mean difference between techniques, and two standard deviations above and below this value. Different symbols indicate which study group each subject was from, as shown in the key. 4C model; four-compartment model, DXA; dual-energy X-ray absorptiometry, BIA; bioelectrical impedance analysis. DX; healthy children, PT; young adults born preterm, OB; obese children, GSD; glycogen storage disease, ALL; acute lymphoblastic leukaemia, CF; cystic fibrosis, ED; eating disorders.

Figure 3. Mean bias and limits of agreement of each ‘simple’ method for measuring fat mass (FM) when compared to the 4C (4 compartment) model: (A) total study population; (B) normal weight group; ((C) overweight group; (D) underweight group.

Difference from 4C FM SDS is calculated by subtracting the 4C FM SDS from that of the simple method. Limits of agreement are calculated as mean bias ±2 standard deviations (SDs). DXA; dual-energy X-ray absorptiometry, BMI; body mass index, SFT; skinfold thickness. Evaluation of SFT as a measurement of FM was carried out using a limited population consisting of study groups DX, PT and CF. The OB, GSD, ALL and ED studies did not include SFT measurement as part of their protocols. Since no subjects were available for the overweight disease group, normal subjects were compared to underweight subjects only.

Figure 4. Mean bias and limits of agreement of each ‘simple’ method for measuring fat-free mass (FFM) when compared to the 4C (4 compartment) model: (A) total study population; (B) normal weight group; ((C) overweight group; (D) underweight group.

Difference from 4C FFM SDS is calculated by subtracting the 4C FFM SDS from that of the simple method. Limits of agreement are calculated as mean bias ±2 standard deviations (SDs). DXA; dual-energy X-ray absorptiometry, BIA; bioelectrical impedance analysis.